1. Field of the Invention
The present invention relates to a modulation circuit, and more particularly to an integrated modulation circuit. This invention can, for example, be applied to a modulation circuit designed as an MMIC (Monolithic Microwave Integrated Circuit). MMIC modulation circuits can, for example, be used for ETC (Electronic Toll Collection) or the like in an. ITS (Intelligent Transit System).
2. Description of the Related Art
The structure of a common conventional modulation circuit is described using the circuit diagram of FIG. 1.
In the modulation circuit depicted in FIG. 1, a carrier S.sub.2 is inputted via a transformer 1, fed to the base of a bipolar transistor 2, and amplified by this bipolar transistor 2. A modulating wave (data signal) S.sub.1 is inputted via a transformer 3 and is applied to the collector of the bipolar transistor 2. The output of the carrier S.sub.2. amplified by the-bipolar transistor 2, is controlled by the modulating wave S.sub.1. Specifically, a modulated wave V.sub.out is generated due to the modulation of the carrier S.sub.2 with the modulating wave S.sub.1. The modulated wave V.sub.out is outputted from the modulation circuit via a transformer 4.
The modulation circuit depicted in FIG. 1 is difficult to design as an MMIC because of the use of the transformers 1, 3, and 4. This is the reason that separate elements are commonly used as the transformers 1, 3, and 4 and as the bipolar transistor 2 of this modulation circuit. Consequently, such a modulation circuit is disadvantageous in that it has considerable dimensions and entails high manufacturing costs.
In addition, the frequency of a carrier S.sub.2 that can be used in such a modulation circuit is approximately several tens of megahertz. The modulation circuit is therefore disadvantageous in that it cannot be used in systems employing other frequency bands. For example, ETC systems operate in a 5.8-GHz band, making it impossible to use this modulation circuit.
The following report describes other types of modulation circuits.
IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, MTT-23, No. 6, June 1975 (USA), C. A. Liechti, p. 461.
In the modulation circuits described in this report, the carrier S.sub.2 is modulated using dual-gate field-effect transistors.
A drawback of these modulation circuits, however, is that power leaks into adjacent channels because of a considerable nonlinearity of modulation characteristics (relation between the modulated wave V.sub.out and the modulating wave S.sub.1).